The present invention relates to silicone diffusion pump fluids and more particularly the present invention relates to silicone diffusion pump fluids which are relatively inexpensive to produce as compared to other silicone diffusion pump fluids.
Diffusion pumps are well known. Such pumps are connected to mechanical vacuum pumps and are used to produce vacuums in 1.times.10.sup.-4 to the 1.times.10.sup.-8 millimeters of Hg pressure at 25.degree. C. Such apparatuses are utilized to form vacuums in many manufacturing operations as well as in many testing facilities. For instance, such diffusion pumps are utilized to form space chambers for the testing of apparatus and equipment that are to be utilized in space operations. In addition, such diffusion pumps are utilized to produce vacuums for the removal of gases from steel in the process of making certain types of steel. Further, it is well known that such diffusion pumps are utilized to produce vacuums so as to facilitate the depositing of thin metallic films on plastics, glass and other substrates. For instance, a diffusion pump is utilized to produce the necessary vacuum for the deposition of thin metallic coating on the internal face of television tubes.
Accordingly, diffusion pumps have a wide use in industry, as well as in laboratories for the testing and production of various articles. The way a diffusion pump works is that it contains a diffusion pump fluid in a chamber which diffusion pump fluid is heated at a constant rate. The diffusion pump fluid is vaporized and the vapor is forced through a jet in the center of an outer chamber containing the gas molecules to be removed. As it leaves the jet the vaporized fluid is forced downward by deflection member against the walls of the outer chamber which are cooled. As a result the vaporized fluid condenses entraining with it as it condenses the gas molecules to be removed. Then the condensed fluid returns to the boiling chamber of the diffusion pump. The gas is driven off by heating and sent to the mechanical pump where it is exhausted. The cooled and degassed diffusion pump fluid is then recirculated to the heated diffusion pump chamber where it is once more used to entrain gases.
It can be appreciated that diffusion pump fluid must have certain necessary properties. Thus, it must be a fluid which has the proper boiling range and will not give off vapors or gases of its own. It is also a fluid which will boil without bumping, that is, it will boil smoothly and has a steep vapor pressure curve. The diffusion pump fluid is also one that desirably can absorb a lot of gas when it is hot and has a very low absorbance of gases at room temperature.
It has been said by the diffusion pump industry that the right boiling temperatures for the fluid in a diffusion pump should be in the neighborhood of 200.degree. to 225.degree. C. at 1-2 millimeters of mercury pressure. The heating rate of the diffusion pump fluid and the diffusion pump is also set at a fixed rate so as to comply with industry standards.
It would be desirable to be able to change diffusion pump fluid in a particular diffusion pump as the vacuum became better, that is, to insert at that point a diffusion pump fluid which would have the capacity to adsorb a further amount of the light gases, but in practice this is not possible. The industry practice has been to utilize one diffusion pump fluid per diffusion pump. Standardization of diffusion pump fluids and diffusion pumps and interchangeability thereof makes it easier for the use of such diffusion pumps in the industry and especially in the standardization of different processes in industry, for instance, in standardization of the process for depositing of the metallic film on the interface of television picture tubes.
As can be appreciated in the operation of a diffusion pump, the diffusion pump fluid is a critical part in the operation of the pump. Accordingly, much time and effort has been expended in the development and selection of the proper types of diffusion pump fluids for diffusion pumps. There have been developed organic fluids for use as diffusion pump fluids. Examples of these are organic esters, chlorinated hydrocarbons, polynuclear aromatics and others. It should be noted that the silicone diffusion pump fluids have an advantage over the organic diesters in that they are more stable at elevated temperatures. The chlorinated hydrocarbons are not desired because of toxicity problems. In addition, the polynuclear aromatics, organic diffusion pump fluids, as well as the polyphenylethers are very expensive and difficult to produce, thus, commanding a very high price in the market. Accordingly, silicone diffusion pump fluids have been developed in the past. It should be noted while such silicone diffusion pump fluids had several advantages in that they were not as toxic as some of the organic materials and were more stable at high temperatures, nevertheless, most of such silicone fluids have turned out to be very expensive. Examples of silicone diffusion pump fluids sold and manufactured by Dow Corning Corporation, Midland, Mich. are as follows: DC-700 which is essentially a light end material. Along with this material there was a comparable material, DC-702, which is a mixture of methyl and methylphenyl cyclicpolysiloxanes. While DC-702, for instance, was utilized in steel mills and was more effective in creating a vacuum than the organic diesters, nevertheless, it has been found by experience that good diffusion pump fluids are produced from a single polymer specie. When the diffusion pump fluid is composed of a mixture of polymer species, such as in DC-700 and DC-702, then the diffusion pump fluid is not very effective or is not as effective as desired because the composition has a low boiling point. In the formulas below, .phi. stands for phenyl.
Accordingly, other diffusion pump fluids that were developed by Dow Corning Corporation were DC-704 of the formula, ##STR2## and DC-705 having the formula, ##STR3## While DC-704 is acceptable and better than DC-702 or DC-700 in its characteristics as a diffusion pump fluid, and the best diffusion pump fluid manufactured and sold by Dow Corning Corporation is DC-705 which is capable of producing a vacuum of up to 1.times.10.sup.-8 millimeters of Hg pressure, however, such diffusion pump fluids such as DC-704 and DC-705 were very expensive to produce because of the esoteric chain-stoppers or terminal siloxy units in the polymer chain that were used to produce the polymer. These esoteric chain-stoppers to some silicone manufacturers were not ones that were obtained as byproducts from normal silicone processes in a silicone manufacturing plant and had to be specially produced. Accordingly, there resulted a high cost for the production of such silicone polymers. As a result, while such silicone polymers were excellent diffusion pump fluids, nevertheless, they were very expensive to produce and commanded a high price at the market place.
Accordingly, it was highly desirable to produce a cheap diffusion pump fluid. One attempt at this was the production of a polymer having the formula, ##STR4## where .phi. stands for phenyl. While having about the same diffusion pump characteristics as DC-702 in terms of creating a vacuum, nevertheless, the above compound was expensive to produce because it was obtained in very low yields by the processes normally used to prepare it. Attempts were made to reduce the cost of the production of such fluids. However, even after repeated attempts at improving the yield of the desired product there only resulted a 30% yield of the desired product. In addition, the product contained trifunctional siloxy cyclicpolysiloxanes which would evaporate from the rest of the fluid when the fluid was used as diffusion pump fluid in a diffusion pump and create problems in its performance. In the course of such research, there was developed a diffusion pump fluid which had good boiling point characteristics. This fluid had the structure, ##STR5## where .phi. stands for phenyl. The difficulty with such diffusion pump fluid while it had good boiling characteristics and good vapor pressure characteristics and it was cheap to produce was that it had a melting point at 46.degree. C. and any normal modifications in the formulations only changed the melting point slightly. Accordingly, in normal operations of a diffusion pump such a diffusion pump fluid would solidify at room temperature and cause problems in handling in the diffusion pump.
Accordingly, it was highly desirable to develop inexpensive silicone diffusion pump fluid which could be made by a simple process utilizing as reactants chlorosilanes normally found or normally produced in a silicone manufacturing plant. It was also desirable to produce inexpensive diffusion pump fluids which could effectively compete with diffusion pump fluids, DC-704 and DC-705, produced by Dow Corning Corporation.
Accordingly, it is one object of the present invention to produce an inexpensive diffusion pump fluid by a simple process which could be utilized as an offset to existing diffusion pump fluids.
It is an additional object of the present invention to provide an efficient process for producing an inexpensive diffusion pump fluid.
It is still an additional object of the present invention to provide for a reverse hydrolysis procedure for producing an inexpensive diffusion pump fluid.
It is yet an additional object of the present invention to provide for a simple process for producing inexpensive silicone diffusion pump fluids which will perform almost as effectively or as effectively as the more expensive diffusion pump silicone fluids.
These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.